The use of direct radiant lamp heating of silicon wafers in Rapid Thermal Processing (RTP) equipment is gaining widespread acceptance. Experimental observations suggest that the lamp heater is a critical factor in achieving an acceptable temperature ramp time, and controllable process uniformity on the wafer. 38 kW lamp modules have been shown to provide good temperature ramping performance resulting in fast throughput and short process time. However, the present uniformity performance of this equipment for high pressure processing (e.g. 650 tort) is unacceptable since typically, the center of the wafer undergoing processing is heated to higher temperatures than the edges of the wafer. This problem shall be explained in more detail with reference to FIGS. 1a and 1b which represent a top view and a side cross-sectional view, respectively, of a prior art lamp heater for optical zone heating of a semiconductor wafer. A source of light comprising 37 lamps are arranged in an axisymmetric 3-zone configuration including a central 2 kW light bulb 2, a middle ring of 12 1 kW light bulbs 4, and an outer ring of 24 1 kW light bulbs 6. Bulb 2 shall be referred to as the central zone lamp. Middle ring of bulbs 4 shall be referred to as the middle zone of lamps and outer ring 6 shall be referred to the outer zone of lamps. The bulbs fit into lamp assembly 3. The radiant power emitted from the bulbs is concentrated by gold plating 8 into process chamber 9 onto silicon wafer 10 (for instance, a 6 inch diameter wafer), through quartz window 12. Quartz window 12 can, for instance, be 10 inches in diameter and 1/2 inch thick. Radiant power will be directed at wafer 10. Heating uniformity is attempted through the manipulation of electrical power supplied to each zone of lamps. Thus selective heating of the outer rim, 11, of wafer 10 is intended relative to central portion 13 of wafer 10 by generating more radiant power from the outer zone of lamps than from the other zone of lamps. However, this lamp heater allows significant optical overlap between the three zones of lamps in terms of incident radiant power on wafer 10. This overlap is the source of the problem with the temperature profile and process uniformity control since central portion 13 of wafer 10 is heated more by the heater relative to the rest of wafer 10, particularly around edge 11. Even in an attempt to prevent this non uniform heating, with the outer zone turned on, and the remaining zones are turned off, significantly more heating of central portion 13 of wafer 10 will still occur relative to edge 11. Differential heating around edge 11 is relatively minor when compared with this problem concerning the heating of central portion 13 of wafer 10. Given the added effects of edge 11 cooling and gas flow dynamics at high pressure in process chamber 9, this problem of central portion 13 heating is very undesirable. It is critical that wafer edge 13 be heated selectively to overcome these effects and allow uniform process results.
Other lamp heaters, such as the G-Squared.TM. lamp, solve the problem by imbedding the bulbs in a water cooled metallic casing where there is no overlap at all between light radiated from individual lights bulbs onto a wafer. This type of lamp heater is in many cases prohibitively expensive, primarily because of the difficulties in manufacturing the lamp hardware.
A need therefore exists to develop a less costly means to solve the foregoing discussed problem.